The present invention relates to the fabrication of Group III-V semiconductor devices and more particularly, to a method for generating a Group III-V semiconductor substrate having a substantially lower defect density than currently available substrates.
Laser diodes that use semiconductor material based on GaN, or other Group III-V semiconductors that emit in the blue and violet regions of the spectrum hold the promise of substantially improving the amount of information that can be stored on an optical disk. To improve the cost effectiveness of such devices, the light-emitting and light-receiving elements together with various unipolar/bipolar transistors, diodes, and passive elements need to be integrated on the same substrate. These additional types of elements are also fabricated using Group III-V semiconductors.
One problem in constructing such integrated circuits is the inability to provide a single crystal Group III-V substrate of sufficient size to accommodate all of the desired devices needed for the integrated circuit. In general, the devices are constructed on a single crystal Group III-V substrate. At present, Group III-V semiconductor single crystal substrates are limited to diameters of several millimeters to one centimeter. These substrates are too small for practical applications. Hence, systems based on the growth of the Group III-V single crystals on a different type of substrate such as sapphire or SiC have been developed. Unfortunately, the lattice constants of these substrates are significantly different from the lattice constants of the Group III-V semiconductor materials. This difference leads to defects when a Group III semiconductor layer is grown on the substrate. Typically 107 to 1011 crystal defects are generated per cm2. These defects limit the performance of the fabricated elements.
Various techniques for reducing these defects have been proposed. For example, A. Usui, H. Sunakawa, A. Sakai, and Yamaguchi, 36 JPN. J. APPL. PHYS., L899 (1997) teach a method for reducing threading dislocations in Group III semiconductors. In this conventional technique, a Group III nitride semiconductor buffer layer is deposited on a non-Group III-V substrate such as sapphire. Next, strips of a dielectric thin film of SiO2 are formed on the surface of the buffer layer. A Group III nitride single crystal layer is then grown. The second Group III nitride layer is seeded from the region of the buffer layer between the dielectric strips and grows out over the strips. It is observed experimentally that the material that grows out over the strips has a significantly lower density of defects. Typically, dislocation densities from around 105 to 107 cmxe2x88x922 are obtained over the dielectric thin film. While this method substantially reduces the density of dislocations, there is still a need to provide further reductions in the dislocation density.
In addition, this method requires that the substrate and buffer layer be removed from the reactor used to grow the Group III nitride layers at least once to deposit the dielectric film strips. The handling problems associated with the removal of the substrate from the reactor, depositing the dielectric strips, and then returning the substrate to the reactor lead to mechanical problems in the buffer layer. As a result, the buffer layer tends to peel off of the underlying substrate.
Broadly, it is the object of the present invention to provide an improved Group III-V single crystal thin film on which to construct Group III-V semiconductor devices.
It is a further object of the present invention to provide a Group III-V single crystal thin film having a lower density of defects than that obtained by growing a Group III-V semiconducting layer over dielectric strips.
It is a still further object of the present invention to provide a method for growing a Group III-V single crystal thin film that does not require that the substrate be removed from the reactor during the growth process.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a substrate for fabricating semiconductor devices based on Group III semiconductors and the method for making the same. A substrate according to the present invention includes a base substrate, a first buffer layer, and a first single crystal layer. The first buffer layer includes a Group III material deposited on the base substrate at a temperature below that at which the Group III material crystallizes. The Group III material is crystallized by heating the buffer layer to a temperature above that at which the Group III material crystallizes to form a single crystal after the Group III material has been deposited. The first single crystal layer includes a Group III-V semiconducting material deposited on the first buffer layer at a temperature above that at which the Group III semiconducting material crystallizes. In one embodiment of the present invention, a second buffer layer and a second single crystal layer are deposited on the first single crystal layer. The second buffer layer includes a Group III material deposited on the first single crystal layer at a temperature below that at which the Group III material crystallizes. The Group III material is then crystallized by heating the buffer layer to a temperature above that at which the Group III material crystallizes to form a single crystal. The second single crystal layer includes a Group III-V semiconducting material deposited on the second buffer layer at a temperature above that at which the Group III semiconducting material crystallizes.